2 Answers
2

This phenomenon has been demonstrated in studies [1][2] which show a significant increase in urine output in the cold. The latter study shows the biggest increase is when a person is initially exposed to the cold rather than when they continue to be exposed or rewarmed up. In the study, the average urine out was 157 mL/hour when a person was initially exposed to the cold, 103 mL/hour when they were being maintained at that cold temperature and 70 mL/hour when they were being rewarmed.

The mechanism however is unclear. This study showed that the mean arterial pressure increases. It is well known that the cold causes our blood vessels to constrict (in order to keep the blood and therefore heat inside our core body). The thought was therefore that our kidneys try to lower our blood pressure by taking more fluid out and emptying it into the bladder. This is called cold-induced diuresis.

Other thoughts include that we lose the water we ingest by sweating (perspiring), pooping (defaecation) and peeing (micturition/urination). In the winter, we don't typically sweat as much, which ISN'T counterbalanced with a lower water intake, so we need to pee more.

Another possibility is that aquaporins (the pores that water comes out from our kidneys) are inhibited by the cold. This means more fluid would be filtered out. However there isn't really much research in this area that I am able to find.

$\begingroup$Interesting possibilities, but some references would be awesome. It would also be interesting to know whether the premise posed in the question is actually true or if it just seems true (perhaps because it's less pleasant to use the bathroom when it's cold, or for some other reason).$\endgroup$
– robFeb 1 '15 at 22:17

$\begingroup$The last paragraph would be especially nice to have a reference for. In warm-blooded animals, it’s not obvious how a cold environment affects the temperature sensed at the level of the aquaporins. I guess you’re positing some more elaborate mechanism via CNS thermoregulation/ADH? (And minor point: aquaporins don’t affect filtering (glomerulus) but reabsorption more distally (collecting duct).)$\endgroup$
– SusanFeb 2 '15 at 9:01

$\begingroup$@rob, as requested I have added two references which demonstrate this effect.$\endgroup$
– AndroidPenguinFeb 4 '15 at 0:22

The blood pressure is thought to be increased by vasoconstriction due to the cold. One source that supports that theory: Sun, Z. American Journal of Physiology -- Regulatory, Integrative, and Comparative Physiology, February 2005; vol 288: pp R433-R439. American Heart Association: "High Blood Pressure."

$\begingroup$Could you elaborate this answer a bit more (e.g., why is blood pressure higher in the cold?). As of now it is more of a comment and comments should not be posted as an answer.$\endgroup$
– AliceD♦Feb 2 '15 at 2:27